Stable tooth whitening gels containing high percentages of hydrogen peroxide

ABSTRACT

An aqueous gel useful for bleaching teeth comprising: (i) water; (ii) polyacrylic acid thickening agent; (iii) hydrogen peroxide bleaching agent; and (iv) aminocarboxylic acid/salt stabilizing agent. Preferably the polyacrylic acid is an easy to disperse carbomer and the gels contain high concentrations (20 to 40% by weight) of hydrogen peroxide. The gels exhibit room temperature stability, both with respect to gel stability and hydrogen peroxide decomposition, sufficient to eliminate the need for constant refrigeration. A method for using the gels to bleach teeth is also disclosed.

RELATED APPLICATIONS

This application is a continuation of and claims priority to U.S. patentapplication Ser. No. 10/386,656 filed Mar. 13, 2003, which is adivisional of and claims priority to U.S. patent application Ser. No.09/428,910 filed Oct. 28, 1999, now U.S. Pat. No. 6,555,020, whichclaims priority to U.S. Provisional Application Ser. No. 60/106,076filed Oct. 29, 1998.

TECHNICAL FIELD

This invention pertains to bleaching gels for use in bleaching teeth.More specifically this invention relates to the production of stableaqueous bleaching gels that contain a very high percentage of hydrogenperoxide and methods for their use in bleaching teeth.

BACKGROUND ART

A gel is a colloid produced by combining a dispersed phase with acontinuous phase (i.e. a dispersion medium or matrix) to produce aviscous, jelly-like, semisolid material. In the dental industry, gelsare utilized as vehicles for applying a variety of dentifrices,bleaching aids, and fluoride compounds to teeth. A “dental bleachinggel” is a gel that carries a bleaching agent that can be safely appliedto teeth.

Hydrogen peroxide has become the bleaching agent of choice for use indental bleaching gels. Hydrogen peroxide is a powerful oxidizer whichserves to bleach the colored materials in the teeth, thereby, producinga whiter appearance.

Unfortunately, at room temperature, hydrogen peroxide will attack thegelling agents used to make the dental bleaching gels. As a result ofthis attack, the gelling agents break down over time. Eventually, thegelling agents break down to such a degree that the gel's viscositybecomes too low to be suitable.

Viscosity is very important to the effectiveness of dental bleachinggels. If the viscosity is too low, the gel will flow uncontrollably fromthe dispensing tube and become difficult to manipulate for the purposesof varying or equalizing the bleaching treatment applied to the teeth.More importantly, if the viscosity is too low, the gel is more likely toflow away from the teeth, resulting in a reduced residence time.Residence time is the time the dental bleaching gel actually contactsthe tooth enamel. The effectiveness of a dental bleaching gel isdirectly proportional to its residence time.

In addition, hydrogen peroxide tends to decompose at room temperature.The rate of this decomposition is dependent upon many factors. Thepresence of various metallic impurities, such as iron, manganese, copperand chromium, catalyze the decomposition. Furthermore, the stability ofhydrogen peroxide decreases with increasing alkalinity and temperature.Because the whitening ability of a dental bleaching gel depends on thehydrogen peroxide concentration, premature decomposition diminishes theability of the gel to whiten.

To combat these problems, dental bleaching gels containing highconcentrations of hydrogen peroxide are generally refrigerated untilimmediately prior to use. Refrigeration slows down the hydrogen peroxideattack on the gelling agent and also slows down hydrogen peroxidedecomposition. However, refrigeration is both expensive andinconvenient.

Various stabilizing agents have been investigated in an attempt todevelop hydrogen peroxide containing dental bleaching gels that arestable at room temperature. Unfortunately, to date, these stabilizingagents have either proven ineffective.

The instant invention solves these stability problems by providing, forthe first time, an effective bleaching gel that is stable at roomtemperature and that contains a large concentration of hydrogenperoxide. There appears to be very little prior art that is relevant tothis invention. The most relevant prior art known to the inventorsincludes the following:

U.S. Pat. Nos. 5,422,073, 5,500,186, 5,593,637 and 5,756,045 teach amethod for disinfecting a contact lens which includes contacting thelens with an isotonic aqueous solution comprising 0.6 to 2 weightpercent tromethamine. Other aspects include adding to the solution achelating agent (preferably disodium EDTA) and/or additionalmicrobicide. The microbicide may be a very low concentration of hydrogenperoxide, e.g., 50 to 200 ppm. The disodium EDTA is not used as astabilizer for the hydrogen peroxide.

U.S. Pat. No. 5,759,440 teaches an aqueous solution (as opposed to agel) of hydrogen peroxide stabilized by incorporation of a compositioncontaining a mixture of an alkali metal pyrophosphate or alkaline earthmetal pyrophosphate with a stabilizer belonging to the category ofaminopolycarboxylic acids. The solution finds application for bleachingtextiles and paper pulps.

U.S. Pat. No. 5,641,386 teaches a process for the bleaching of pulpcomprising the step of bleaching the pulp with hydrogen peroxide and aneffective amount of at least one biodegradable1-amino-alkane-1,1-diphosphate chelating agent to enhance the bleachingof the pulp.

U.S. Pat. No. 4,812,173 teaches stabilized hydrogen peroxidedisinfecting solutions containing diphosphoric acids (such ashydroxyethylidene diphosphoric acid), and glycerin.

U.S. Pat. No. 5,248,389 teaches a process for peroxygen bleaching ofhigh yield pulp in which sodium carbonate replaces sodium hydroxide andsodium silicate.

U.S. Pat. Nos. 5,098,303, 5,376,006 and 5,725,843 teach high viscositysustained release dental compositions, such as tooth bleaching orfluoride compositions, for treating tooth surfaces. The sustainedrelease dental compositions include a high carboxypolymethyleneconcentration (typically greater than 3.5%) which results in very highviscosity. The bleaching gels can contain from about 3 to about 20%carbamide peroxide, preferably about 4% to about 15% carbamide peroxide.Alternatively, the bleaching gels can contain hydrogen peroxide in apreferred range of from about 2% to about 10%.

U.S. Pat. No. 4,226,851 teaches a stable dental hygiene compositioncomprising a mixture of hydrogen peroxide and zinc chloride. The mixtureis stabilized by the addition of water soluble vitamin E.

Ultradent, produced by Ultradent Products of South Jordan, Utah, offersa 35% hydrogen peroxide bleaching gel product called Opalescence Xtra.The package contains a prominent warning which states “REFRIGERATIONREQUIRED!” Similar warnings appear in two places on the package insert.Opalescence Xtra is a gel that is red in color due to the presence ofβ-carotene. Opalescence Xtra turns into a colorless, runny liquid inless than two weeks when stored at room temperature.

None of the aforementioned references describes the stabilization ofgels with respect to both gel stability and hydrogen peroxide stability.

SUMMARY OF THE INVENTION

This invention is directed to aqueous gels useful for bleaching teethcomprising: (i) water; (ii) polyacrylic acid thickening agent; (iii) atleast one bleaching agent selected from the group consisting of hydrogenperoxide and compounds that release hydrogen peroxide in water; and (iv)aminocarboxylic acid/salt stabilizing agent. Preferably the gel alsocomprises (v) a neutralizing agent that serves to neutralize thepolyacrylic acid. Ideally, these gels are fast acting bleaching gelsthat contain 20 to 50% by weight bleaching agent.

The combination of polyacrylic acid thickener and the aminocarboxylicacid/salt stabilizer provides a gel that can be loaded with hydrogenperoxide and remain sufficiently stable to provide a suitable gel (froma viscosity standpoint) after 4 to 12 weeks storage at room temperature.In addition, the gels exhibit little hydrogen peroxide decomposition.

These gels may be applied by a method comprising the following steps:(a) protecting the soft tissue surrounding the teeth with a covering;(b) applying the aforementioned aqueous gel to the teeth; and (c)removing the gel. Preferably, prior to removal, the gel is treated witha laser or heat lamp to accelerate bleaching.

DISCLOSURE OF THE INVENTION

As stated, the instant invention is directed to aqueous gels thatcontain hydrogen peroxide and that are stable at room temperature for upto twelve weeks. These gels comprise: (i) water; (ii) thickening agent;(iii) bleaching agent; (iv) stabilizing agent; and, optionally, (v)neutralizing agent.

Water is the principle component of the aqueous gels. Preferably, wateris present in an amount over 50% by weight. More preferably, water makesup the remainder of the gel after removal of the thickening agent,bleaching agent, stabilizing agent and neutralizing agent.

The thickening agent (a.k.a. gelling agent) is present in an amountranging from 0.25% to 3% by weight of the aqueous gel. The thickeningagent is a “polyacrylic acid” which means that it is selected fromacrylic acid homopolymers and copolymers comprising 90% or more, byweight, polymerized acrylic acid units. The preferred acrylic acidmonomer used to make these thickeners is the actual compound “acrylicacid.” However, other acrylic acids can also be employed, e.g.methacrylic acid and C₁₋₄ alkyl substituted acrylic acid. Othercomonomers that may be present in the polymer chain include 10% byweight or less long chain alkyl esters of acrylic acid.

Suitable thickeners include the crosslinked polyacrylic resins sold byB.F. Goodrich under the tradename Carbopol®. The USP-NF, BritishPharmacopoeia, United States Adopted Names Council (USAN), and Cosmetic,Toiletries and Fragrance Association (CTFA) have adopted the generic(i.e. non-proprietary) name “carbomer” for the Carbopol® homopolymers.The Japanese Pharmaceutical Exipients list Carbopol® homopolymers as“carboxyvinyl polymer” and “carboxy polymethylene.” All of thesepolymers have the same acrylic acid backbone. The main differences arerelated to the presence of comonomer and crosslink density. Thesepolymers are either homopolymers of acrylic acid crosslinked with allylsucrose, polyalkyl ethers of divinyl glycol, or allyl pentaerythritol orsimilarly crosslinked copolymers of acrylic acid with minor levels oflong chain alkyl acrylate comonomers. These polymers swell in water upto 1000 times their original volume (and ten times their originaldiameter) to form a gel when exposed to a pH environment above 4.0-6.0.Carbopol® thickeners are highly resistant to hydrolysis and oxidationunder normal conditions.

Preferred thickeners include Carbopol® ETD™ 2001, Carbopol®ETD™2020, andCarbopol® ETD™ 2050. These “easy-to-disperse (ETD™)” thickeners arehomopolymers or copolymers of acrylic acid, produced using apolymerization aid, and crosslinked with a polyalkenyl polyether.Carbopol® ETD™ 2001, Carbopol® ETD™ 2020, and Carbopol® ETD™ 2050 areeasier to disperse and mix than other Carbopol® products. The thickenerswet quickly and thereby minimize lumping. By “wet” it is meant that thewhite particles of polymer fully disappear (disperse) into the mixture.The thickeners also hydrate slowly and have a lower viscosity prior toneutralization than other Carbopol® products. Because of the fastwetting nature and low viscosity of the thickeners, vigorous agitationis not necessary to disperse them. This is important to the instantinvention since vigorous agitation is undesirable because it induceshydrogen peroxide decomposition. The fast wetting nature of thethickeners also aids handling. Once the ETD™ resins are neutralized,they provide the type of highly efficient thickening for which Carbopolresins are known.

The most preferred thickener is Carbopol® Ultrez™ 10. This thickener isan exceptionally easy-to-disperse polymer that wets even more quicklythan the Carbopol ETD™ resins. In fact, Carbopol® Ultrez™ 10 wetswithout any stirring. For example, a 500 gram dispersion at 0.5% resin(2.5 grams) will take only about 5 minutes to completely wet withoutmixing. This decreases the time and effort necessary to achieve alump-free dispersion.

The bleaching agent utilized in the aqueous gel is present in an amountranging from 3 to 50%, preferably 20 to 50%, more preferably 30-40%, andmost preferably 35% by weight of the aqueous gel. Higher amounts ofbleaching agent are preferred so that the gel may serve as a “fastacting bleaching gel” capable of bleaching teeth with only one or twoapplications.

The bleaching agent may be selected from hydrogen peroxide (H₂O₂) or anycompound that yields hydrogen peroxide when placed in an aqueous medium.In example, carbamide peroxide (CO(NH₂)₂H₂O₂) generates hydrogenperoxide when placed in water. Other names for carbamide peroxideinclude urea peroxide, urea hydrogen peroxide, hydrogen peroxidecarbamide, and perhydrol urea.

The stabilizing agent utilized in the aqueous gel is present in anamount ranging from 0.05 to 0.5% by weight of the aqueous gel. An amountof approximately 0.15% stabilizer is preferred. The stabilizing agent isselected from aminocarboxylic acids and salts thereof. Preferredstabilizers are selected from aminocarboxylic acids and alkali and/oralkali earth metal salts thereof. Suitable aminocarboxylic acids includetrans-1,2-cyclohexylene dinitrilotetraacetic acid (CDTA),ethylenediamine tetraacetic acid (EDTA), N-(2-hydroxyethyl)ethylenediamine triacetic acid (HEDTA), Nitrilotriacetic acid (NTA),diethylene triamine pentaacetic acid (DTPA), triethylene tetraaminehexaacetic acid (TTHA), and ethyleneglycol bis (2-aminoethylether)tetraacetic acid (GEDTA). The most preferred stabilizers include CDTA,CaNa₂EDTA, Na₂EDTA, Na₄EDTA, HEDTA, and Na₃HEDTA.

The combination of polyacrylic acid thickener and aminocarboxylicacid/salt stabilizer provides a gel that can be loaded with hydrogenperoxide and that is stable in the sense that it maintains a suitablegel for 4 to 12 weeks at room temperature. For example, a 3.4/1 ratio ofpolyacrylic acid/CaNa₂EDTA yields an aqueous gel that is stable for 12weeks, a 3.5/1 ratio of polyacrylic acid/CDTA yields an aqueous gel thatis stable for 7 weeks, and a 0.3/1 ratio of polyacrylic acid/Na₂EDTAyields a gel that is stable for 41/2 weeks. By “suitable gel” it ismeant that when a drop of the gel is placed on the surface of a flatglass plate and the surface is turned vertically, the gel dropletremains in place on the plate surface. It is not certain why the resultsobtained using different aminocarboxylic acid/salt stabilizers vary.What is certain is that the polyacrylic acid thickener is more resistantto attack than many other thickeners and that the presence of anaminocarboxylic acid/salt stabilizer can enhance this stability manyfold.

In addition, the stabilizer prevents substantial hydrogen peroxidedecomposition at room temperature. Hydrogen peroxide loss in gelsutilizing polyacrylic acid thickeners and aminocarboxylic acid/saltstabilizers is less than 0.05% by weight per day.

The end result is that the aqueous gels can now be produced that havecommercially viable shelf-lives at room temperature. Thus, constantrefrigeration, which is both expensive and inconvenient, is no longernecessary.

In addition to the aforementioned components, a neutralizing agent maybe added to the aqueous gel. The presence of a neutralizing agent ispreferred since it serves to further thicken the system. Theneutralization agent ionizes the polyacrylic acid and generates negativecharges along the backbone of the polymer. Repulsions of like chargesthen cause uncoiling of the polymer into an extended structure. Thisreaction is rapid and gives instantaneous thickening.

The inorganic and organic neutralizing agents which may be employed arebases. Suitable bases include alkali metal hydroxides and ammoniumhydroxide, carbonates, alkoxides, oxides, peroxides, superoxides, andwater soluble organic amines. Amino acids such as β-alanine and lysinecan also be used for neutralization and viscosity modification.Preferred bases include sodium hydroxide, potassium hydroxide, ammoniumhydroxide, triethanolamine (TEA), aminomethyl propanol (AMP),2-amino-2-hydroxymethyl-1,3-propanediol (Tromethamine),tetrahydroxypropyl ethylenediamine, and tris(hydroxymethyl)aminomethane(TRIS). The amount of base utilized is the amount of base necessary tofully neutralize the polyacrylic acid thickener in the aqueous gel. Thisamount will vary considerably depending on the nature of the base andthe amount of polyacrylic acid. In example, the following Table 1 setsforth the amount of different bases required to neutralize polyacrylicacid to an appropriate pH of 6.0-7.0:

TABLE 1 Relative ratio of base to one part Base polyacrylic acid byweight Sodium hydroxide (18% solution) 0.5 Potassium hydroxide (18%solution) 0.5 Ammonium hydroxide (28% solution) 0.3 Triethanolamine(TEA) 2.0 Tromethamine (2-Amino-2-Hydroxymethyl- 2.0 1,3-propandiol)Aminomethyl propanol (AMP) 1.5 Tetrahydroxypropyl ethylene diamine 2.0

The dental bleaching gel of the instant invention can be applied to theteeth in a number of ways. In example, the gel can be applied to theteeth using a brush, syringe, tray, or any other application means.

In a typical treatment process, the soft tissues surrounding the teethare first covered with a protecting device, e.g. a ligated rubber dam.This is important because the more hydrogen peroxide a dental bleachinggel contains, the more likely it is to bum the soft tissue upon contact.Dental bleaching gels containing at least 30% by weight hydrogenperoxide will immediately bum any soft tissue they contact, quicklyturning the tissue white.

Next a brush, needle, or some other delivery system is utilized to placethe dental bleaching gel described above in contact with the teeth onewishes to bleach. Most patients only request treatment on the labialsurfaces of the 6 to 8 front teeth which show most prominently when onesmiles.

The dental bleaching gel is then allowed to remain in contact with theteeth for a period of time ranging anywhere from 5 minutes to one hour.Preferably, however, this contact period ranges from 20 to 30 minutes.As stated earlier, the bleaching effect of any dental bleaching gel isdirectly proportional to this residence time.

The bleaching effect of the hydrogen peroxide in a given period of timecan be amplified by applying a heat lamp or laser light to the dentalbleaching gel once it is in place on the teeth. The heat and light serveto increase the rate of bleaching of the hydrogen peroxide, providing ashorter period of time for whitening the teeth.

Once the treatment is done, the gel is removed with a gauze or someother means. The patient's mouth is then thoroughly cleaned with waterand suction. When the dental bleaching gel comprises at least 20% byweight hydrogen peroxide, only one or two such treatments are necessary.

The following examples are illustrative of the invention:

EXAMPLES 1-19

Various blends of powdered stabilizer (or aqueous solution in the caseof Versenol 120) and powdered thickening agent were weighed in a plasticor glass beaker. Aqueous hydrogen peroxide was then added to the blendsin the amount of 35% (w/w). The mixtures were gently stirred until thepowdered ingredients fully dissolved. This completed the procedure inthe blends wherein carboxymethylcellulose, xanthan gum, and carrageenanwere used as the thickeners. In the blends containing polyacrylic acidas the thickener, triethanolamine was also added. All of the gels werethen packaged in 10 mL polyethylene syringes. The composition of eachblend formulation is set forth in the following Table 2:

TABLE 2 Tri- ethanol- Gelling agent Stabilizer 35% H₂O₂ amine Example(grams) (grams) (grams) (grams)  1 0.102 — 20.00 0.050 Polyacrylicacid*¹  2 0.102 0.030 20.00 0.050 Polyacrylic acid CaNa₂EDTA*⁵  3 0.4120.030 20.00 — Xanthum Gum*² CaNa₂EDTA  4 0.514 0.031 20.00 — Xanthum GumCaNa₂EDTA  5 0.612 0.030 20.00 — Carboxymethyl CaNa₂EDTA cellulose*³  61.262 0.030 20.00 — Sodium CaNa₂EDTA carrageenan*⁴  7 0.102 0.0294 20.000.150 Polyacrylic acid CDTA*⁶  8 0.102 0.0310 20.00 0.100 Polyacrylicacid Na₂EDTA  9 0.102  .0309 20.00 0.100 Polyacrylic acid Na₄EDTA 100.102 0.0972 20.00 0.100 Polyacrylic acid Na₃HEDTA*⁷ 11 0.102 0.10 20.000.100 Polyacrylic acid Vitamin E*⁸ and 0.030 CaNa₂EDTA 12 0.1025 0.031220.0189 0.250 Polyacrylic acid Na₃NTA*⁹ 13 0.1021 0.0318 20.0366 0.200Polyacrylic acid Na₂NTA 14 0.1032 0.0309 20.0029 0.050 Polyacrylic acidNaCaHEDTA 15 0.1022 0.0370 20.0305 0.200 Polyacrylic acid NaCa₂DTPA*¹⁰16 0.1020 0.0300 20.0000 0.300 Polyacrylic acid NaCaNTA 17 0.1026 0.031120.0126 0.200 Polyacrylic acid Ca₃TTHA*¹¹ 18 0.1019 0.0311 20.0339 0.100Polyacrylic acid Na₂Ca₂TTHA 19 0.1024 0.0313 20.0032 0.100 Polyacrylicacid Na₂CaGEDTA*¹² *¹Available from B.F. Goodrich and called Ultrez 10*²Available from Aldrich *³Available from Van Waters & Rogers, Inc.*⁴Available from FMC Corporation *⁵EDTA = Ethylenediamine tetraaceticacid *⁶CDTA = trans-1,2-cyclohexylene dinitrilo tetraacetic acid *⁷HEDTA= N-(2-hydroxyethyl)ethylenediamine triacetic acid which is available asVersenol 120 from Dow Chemical Corp. *⁸Vitamin E = (DL-α-Tocopherol) andis available from Fluka Chemie *⁹Nitrilotriacetic acid *¹⁰DTPA =Diethylene triamine pentaacetic acid *¹¹TTHA = Triethylene tetraminehexaacetic acid *¹²GEDTA = Ethyleneglycol bis (2-aminoethylether)tetraacetic acid

Gel stability was evaluated by placing a drop of each gel onto thesurface of a flat glass plate. The plate was then tilted so that theflat surface was vertical. Gels were considered unsatisfactory when thegel droplet would not remain in place on the plate surface.

Hydrogen peroxide stability was evaluated by iodometric titration. Formore information concerning this test see page 854 of “QuantitativeChemical Analysis,” 4th edition, 1969, written by I. M. Kolthoff, E. B.Sandell, E. J. Meehan and Stanley Bruckstein, and published by TheMacmillan Company/Collier-Macmillan Limited in London, which is hereinincorporated by reference. Peroxide concentrations as a function of timewere fitted to a zero order kinetic model, and the zero order rateconstant (% peroxide loss per day) was used to measure peroxidestability. The tendency of the gels to produce oxygen during storage wasalso observed. This was done in two ways. First, any displacement of thesyringe plunger was noted during each sampling interval. Second, any gelthat expelled from the syringe tip was also noted.

The results of these stability studies are set forth in the followingTable 3, wherein each example corresponds to a like numbered example inTable 2:

TABLE 3 Days as a satisfactory k Example gel (% H₂0₂ per day)Self-expelling 1 7 0.04 Yes 2 84 −0.004 No 3 28 0.0002 No 4 28 −0.01 No5 7 −0.01 Yes 6 7 −0.05 No 7 48 −0.004 No 8 33 −0.05 Yes 9 57 −0.01 No10 44 0.01 Yes 11 18 0.01 No 12 8 −0.001 ? 13 8 −0.01 ? 14 <5 −0.03 No15 3 — No 16 <3 — Yes 17 <7 −0.01 No 18 <7 −0.02 No 19 <3 — No

Examples 2-6 demonstrate the variations in stability that result fromusing different gelling agents with identical amounts of stabilizer(CaNa₂EDTA) and identical amounts of hydrogen peroxide. As can be seen,the polyacrylic acid thickening agent Ultrez 10 is by far the bestgelling agent from the standpoint of gel stability, although xanthan gumprovides a gel with reasonably good stability. Carboxymethylcelluloseand carageenan were not satisfactory. A comparison of Examples 1 and 2demonstrates that the presence of the CaNa₂EDTA stabilizer greatlyenhances the gel stability provided by polyacrylic acid thickeningagents.

Examples 7-19 demonstrate how the stability of gels utilizingpolyacrylic acid gelling agents varies depending on the quantity andtype of aminocarboxylic acid stabilizer employed. As can be seen, thebest stabilizers are ETDA and its salts, CDTA, and HEDTA. Thesestabilizers form gels which have good gel stability, good hydrogenperoxide stability and a low tendency to produce oxygen gas. Note thatsome of the gels have positive zero order constants for hydrogenperoxide decomposition. This is thought to be due to a very low rate ofhydrogen peroxide decomposition in combination with a much higher rateof water evaporation.

While the invention has been described in conjunction with the specificembodiments outlined above, it is evident that many alternatives,modifications, and variations will be apparent to those skilled in theart. Accordingly, the preferred embodiments of the invention areintended to be illustrative and not limiting. Various changes may bemade without departing from the spirit and scope of the invention asdefined in the claims.

What is claimed is:
 1. A method for bleaching teeth comprising thefollowing steps: (a) protecting the soft tissue surrounding the teethwith a covering; (b) applying an aqueous dental bleaching gel to theteeth that comprises: (i) water; (ii) a polyacrylic acid thickeningagent; (iii) at least one bleaching agent selected from the groupconsisting of hydrogen peroxide and compounds that release hydrogenperoxide in water; and (iv) a stabilizing agent selected from the groupconsisting of EDTA, CDTA, and a salt thereof, wherein said bleachingagent is present in an amount ranging from about 20% to about 50% byweight, and wherein said polyacrylic acid thickening agent and saidstabilizing agent are added in a weight ratio within the range of about3.3:1 to about 3.5:1 that maintains said aqueous gel in a suitable gelform for 4 to 12 weeks at room temperature in a syringe plunger withoutself-expelling; and (c) removing the dental bleaching gel.
 2. The methodof claim 1 wherein the dental bleaching gel remains in contact with theteeth for a period of 5 to 60 minutes.
 3. The method of claim 2 whereinthe dental bleaching gel is applied to the teeth and then treated with alaser light or heat lamp prior to removal.